In digital communication, a Quadrature Amplitude Modulation (QAM) scheme that uses both phase information and amplitude information to identify data is known as a modulation/demodulation scheme for efficient data transmission. Recently, an increase in a modulation multilevel number has been desired along with a demand for increasing a capacity of a communication system. However, there is a problem that when the modulation multilevel number is increased, a transmission error probability increases due to noise, thereby decreasing the noise immunity. In particular, in a transmission apparatus and a reception apparatus employing a modulation scheme such as the QAM scheme, phase noise mainly caused by a Local Oscillator (LO) becomes a factor to increase uncertainty of phase information and remarkably degrade a Bit Error Rate (BER).
For example, if a phase error occurs due to phase noise in a communication system using a multilevel QAM scheme having 256 or more signal points, the bit error rate increases, and the reliability of data communication decreases. In such a communication system, it is necessary to estimate the phase error caused by the phase noise with high accuracy and then compensate it in order to perform highly reliable data communication. Further, in a communication system using a multilevel QAM scheme or the like, it is necessary to improve tolerance to errors caused by factors other than the phase noise such as thermal noise.
As a demodulation apparatus that can improve error tolerance, a demodulation apparatus that includes a QAM symbol demapping apparatus which performs phase error compensation using a Phase Lock Loop (PLL) and outputs a bit sequence reflecting likelihood information at a subsequent stage of the PLL, and an error correction decoder which inputs likelihood information and performs error correction processing is known. An example of the QAM symbol demapping apparatus is disclosed in, for example, Patent Literature 1.
However, in the above demodulation apparatus, a sufficiently and satisfactory bit error rate characteristic may not be achieved because of a magnitude of the phase noise included in a baseband signal output from a detector or deteriorated accuracy of phase detection due to thermal noise etc. In order to address this problem, Patent Literature 2 and 3 discloses a technique for improving the accuracy of phase error compensation by adaptively adjusting a bandwidth of a loop filter in a phase lock loop.
In addition to the above-described method of using the phase lock loop, a method of periodically embedding a known signal (a pilot signal) in a transmission signal and compensating the phase noise using this known signal is known. A general principle related to improvement of communication reliability using a pilot signal is described in, for example, Non Patent Literature 1. A method of using the pilot signal to improve the accuracy of the phase noise compensation method is described in, for example, Non Patent Literature 2, 3, and 4.
FIG. 10 is a block diagram showing a configuration example of a phase noise compensation apparatus which compensates phase noise using a pilot signal. Referring to FIG. 10, a phase noise compensation apparatus 200 includes a FIFO (First-In First-Out) memory 201, a phase detector 202, an interpolation filter 203, a phase rotator 204, and a switch 205.
In the phase noise compensation apparatus 200, a reception symbol corresponding to a reception baseband signal is input to the FIFO memory 201. When the reception symbol is a pilot symbol corresponding to a known pilot signal inserted at a transmission side, the reception symbol is also input to the phase detector 202 through the switch 205. The phase detector 202 detects a phase component of the reception pilot symbol and outputs it to the interpolation filter 203.
The interpolation filter 203 includes a selector 206, a register 207, a multiplier 208, and a tap coefficient update apparatus 209. The interpolation filter 203 performs weighted interpolation processing on a plurality of reception pilot symbols and estimates phase noise in the reception symbol between the pilot symbols. The phase rotator 204 rotates a phase of the reception symbol based on phase information output by the interpolation filter 203 to compensate the phase noise in the reception symbol. A phase noise compensation method using such a pilot signal is disclosed in, for example, Patent Literature 4.